netdev - [Patch bpf-next v2] bpf: clear percpu pointers in bpf_prog_clone

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Date:   Wed, 17 Feb 2021 16:16:47 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     netdev@...r.kernel.org
Cc:     bpf@...r.kernel.org, Cong Wang <cong.wang@...edance.com>,
        Jiang Wang <jiang.wang@...edance.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: [Patch bpf-next v2] bpf: clear percpu pointers in bpf_prog_clone_free()

From: Cong Wang <cong.wang@...edance.com>

Similar to bpf_prog_realloc(), bpf_prog_clone_create() also copies
the percpu pointers, but the clone still shares them with the original
prog, so we have to clear these two percpu pointers in
bpf_prog_clone_free(). Otherwise we would get a double free:

 BUG: kernel NULL pointer dereference, address: 0000000000000000
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 0 P4D 0
 Oops: 0000 [#1] SMP PTI
 CPU: 13 PID: 8140 Comm: kworker/13:247 Kdump: loaded Tainted: G                W    OE
  5.11.0-rc4.bm.1-amd64+ #1
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
 test_bpf: #1 TXA
 Workqueue: events bpf_prog_free_deferred
 RIP: 0010:percpu_ref_get_many.constprop.97+0x42/0xf0
 Code: [...]
 RSP: 0018:ffffa6bce1f9bda0 EFLAGS: 00010002
 RAX: 0000000000000001 RBX: 0000000000000000 RCX: 00000000021dfc7b
 RDX: ffffffffae2eeb90 RSI: 867f92637e338da5 RDI: 0000000000000046
 RBP: ffffa6bce1f9bda8 R08: 0000000000000000 R09: 0000000000000001
 R10: 0000000000000046 R11: 0000000000000000 R12: 0000000000000280
 R13: 0000000000000000 R14: 0000000000000000 R15: ffff9b5f3ffdedc0
 FS:    0000000000000000(0000) GS:ffff9b5f2fb40000(0000) knlGS:0000000000000000
 CS:    0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000000 CR3: 000000027c36c002 CR4: 00000000003706e0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 Call Trace:
    refill_obj_stock+0x5e/0xd0
    free_percpu+0xee/0x550
    __bpf_prog_free+0x4d/0x60
    process_one_work+0x26a/0x590
    worker_thread+0x3c/0x390
    ? process_one_work+0x590/0x590
    kthread+0x130/0x150
    ? kthread_park+0x80/0x80
    ret_from_fork+0x1f/0x30

This bug is 100% reproducible with test_kmod.sh.

Reported-by: Jiang Wang <jiang.wang@...edance.com>
Fixes: 700d4796ef59 ("bpf: Optimize program stats")
Fixes: ca06f55b9002 ("bpf: Add per-program recursion prevention mechanism")
Cc: Alexei Starovoitov <ast@...nel.org>
Cc: Daniel Borkmann <daniel@...earbox.net>
Signed-off-by: Cong Wang <cong.wang@...edance.com>
---
 kernel/bpf/core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 0ae015ad1e05..aa1e64196d8d 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1118,6 +1118,8 @@ static void bpf_prog_clone_free(struct bpf_prog *fp)
 	 * clone is guaranteed to not be locked.
 	 */
 	fp->aux = NULL;
+	fp->stats = NULL;
+	fp->active = NULL;
 	__bpf_prog_free(fp);
 }
 
-- 
2.25.1